1. Field of the Invention
The invention relates to field ionization sources and field ion guns for producing intense, high resolution ion beams.
2. Brief Description of the Prior Art
The desirability of an ion gun capable of issuing stable, high current, high resolution focused ion beams has increased with the increased interest in and use of ion beams in applications such as scanning transmission ion microprobes, ion-probe microanalysis, fine ion beam sputtering and ion implantation of doped regions into semiconductor integrated circuits. Present limitations of photolithography in integrated circuit fabrication and present limitations in integrated circuit mask making procedures have also resulted in an increased interest in such ion beams. See "Focused Ion Beams in Microfabrication" by R. L. Seliger and W. P. Fleming, Journal of Applied Physics, Vol. 45, No. 3, March 1974, page 1416-1422; also see "Proton Scanning Microscopy: Feasibility and Promise", by Riccardo Levi-Setti, Proceedings of the Seventh Annual Scanning Electron Microscope Symposium, ITT Research Institute, Chicago, Ill., April 1974.
Field ionization sources have been utilized to produce substantially brighter, higher resolution ion beams than ion beams which are produced by conventional duo-plasmatron ion sources. The field ionization source is unique in that the apparent or virtual source size is very small, being of the order of 10 Angstroms. As a consequence, the brightness of the field ionization source can be very great. The principle of operation of the field ionization source is that when a molecule is placed in a very high electric field (of the order of 10.sup.8 volts/cm) there is an appreciable probability that the molecule will be ionized. In a field ionization source an electric field of sufficient strength to ionize the molecules may be created at the tip of a field emitter with a tip radius of approximately 1000 Angstroms by applying a high voltage, e.g., 10,000 volts, to the field emitter, (hereinafter referred to simply as the emitter.)
A stable "built up" tip having a very small effective radius for thermal field emission cathodes is described in my U.S. Pat. No. 3,817,592. Several basic sources of molecules to be ionized have been utilized, including gases, liquid films condensed from such gases onto the emitter, and the emitter material itself. If the best performances of conventional (duo-plasmatron) ion sources are extrapolated to approximately the 500 Angstrom beam diameter range, the ion beam current is approximately 10.sup.-12 amperes. Ion beams characterized by this level of current lack sufficient brightness to perform many useful operations, such as high resolution sputtering or implanting, at a sufficiently high rate to be of commercial importance. Likewise, field ionization sources or field ion microprobes do not produce narrow, high resolution beams of sufficient intensity to perform many useful operations at a sufficiently high rate to be of commercial importance.
Accordingly, a broad object of the invention is to provide a field ion gun having increased brightness for producing high resolution ion beams.
Another object of the invention is to provide an improved field ionization source by utilizing a built up emitter to increase brightness along the emitter axis.
Chemical activity at the emitter surface may be greatly accelerated by high electric fields; accordingly, certain combinations of emitter material and ionizable material have been found to be undesirable. For example, water molecules attack tungsten very rapidly and will destroy or substantially deteriorate the characteristics of an emitter within a few minutes if a high magnitude electric field is applied and the ambient pressure is of the order of 10.sup.-4 torr. To avoid this problem, it has heretofore been necessary to use expensive ultra-high vacuum systems designed to prevent water contamination.
Accordingly, another object of the invention is to provide a field ionization source having increased resistance to field-induced chemical etching of the emitter.
Ion beams produced by prior art devices have been characterized by an undesirably large energy spread, i.e., by a wide energy distribution of the ions in the ion beam. This reduces the resolution of the focused ion beam because of the inherent chromatic aberration of any lens system. The lens system focuses (i.e., deflects) different energy ions by a different extent, thereby resulting in poor resolution of the focused ion beam.
Accordingly, another object of the invention is to provide a field ionization source producing an ion beam characterized by a narrow energy spread and to achieve this object by controlling the temperature and pressure of a gaseous, atomic or molecular source to form a liquid film on the field emitter, thereby increasing the available supply of ionizable material.